Method and apparatus for demodulating signals processed in a transmit diversity mode

ABSTRACT

Demodulator architectures for processing a received signal in a wireless communications system. The demodulator includes a number of correlators coupled to a combiner. Each correlator typically receives and despreads input samples (which are generated from the received signal) with a respective despreading sequence to provide despread samples. Each correlator then decovers the despread samples to provide decovered “half-symbols” and further demodulates the decovered half-symbols with pilot estimates to generate correlated symbols. The decovering is performed with a Walsh symbol having a length (T) that is half the length ( 2 T) of a Walsh symbol used to cover the data symbols in the transmitted signal. The combiner selectively combines correlated symbols from the assigned correlators to provide demodulated symbols. One or more correlators can be assigned to process one or more instances of each transmitted signal. The pilot estimates used within each assigned correlator to demodulate the decovered half-symbols are generated based on the signal instance being processed by that correlator.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

[0001] The present Application for Patent is a Continuation Applicationclaiming priority to patent application Ser. No. 09/594,466 entitled“Method and Apparatus for Demodulating Signals Processed in a TransmitDiversity Mode” filed Jun. 14, 2000, having a common assignee with thepresent application and hereby expressly incorporated by referenceherein.

BACKGROUND OF THE INVENTION

[0002] I. Field of the Invention

[0003] The present invention relates to data communications. Moreparticularly, the present invention relates to method and apparatus forefficiently demodulating signals that have been processed andtransmitted in a diversity mode.

[0004] II. Description of the Related Art

[0005] In a typical digital communications system, data is processed,modulated, and conditioned at a transmitter unit to generate a modulatedsignal that is then transmitted to one or more receiver units. The dataprocessing may include, for example, formatting the data into aparticular frame format, encoding the formatted data to provide errordetection and/or correction at the receiver unit, channelizing (i.e.,covering) the coded data, and spreading the channelized data over thesystem bandwidth. The data processing is typically defined by the systemor standard being implemented.

[0006] At the receiver unit, the transmitted signal is received,conditioned, demodulated, and digitally processed to recover thetransmitted data. The processing at the receiver unit is complementaryto that performed at the transmitter unit and may include, for example,despreading the received samples, decovering the despread samples togenerate decovered symbols, and decoding the decovered symbols.

[0007] In some communications systems, data is processed and redundantlytransmitted over two (or possibly more) antennas to provide transmitdiversity. The processing may include, for example, covering the datafor each antenna with a particular channelization code (e.g., aparticular Walsh symbol). In some systems, the data for one or moreantennas may also be reordered prior to the channelization. Due tomultipath and other phenomena, the transmitted signals may experiencedifferent path conditions and may arrive at the receiver unit atdifferent times. If the transmit antennas are spaced sufficiently farapart, then the received signals from the antennas tend to fadeindependently. Each transmitted signal may also reach the receiver unitvia multiple signal paths. The receiver unit is then required toreceive, track, and process one or more instances of each transmittedsignal, and to combine the results from the processed signal instancesto recover the transmitted data. On the downlink, the processingtypically includes tracking a pilot that has been transmitted along withthe data, and using the recovered pilot to demodulate data samples.

[0008] The signal processing (e.g., demodulation) to process multipletransmitted signals, and multiple instances of such signals, can becomplicated. Moreover, transmit diversity is typically provided on thedownlink, and user terminals are required to support such a mode. Theuser terminals are typically more impacted by complexity and costsconsiderations. Therefore, techniques that can be used to efficientlydemodulate signals that have been processed and transmitted in adiversity mode are highly desirable.

SUMMARY OF THE INVENTION

[0009] The present invention provides demodulator architectures,demodulators, and receiver units for processing signals that have beenprocessed and transmitted in a transmit diversity mode. When operatingin the transmit diversity mode, data symbols are typically covered witha channelization code (e.g., a Walsh symbol) having a length (2T) thatis twice the length (T) of the channelization code used to cover thedata symbols in the non-transmit diversity mode. The demodulatorarchitectures of the invention exploit this property and perform partialprocessing (e.g., despreading, decovering, pilot demodulation, or acombination thereof) on each fraction of a channelization symbol periodof 2T. The processed “partial-symbols” are then appropriately combinedto generate the demodulated symbols. By performing partial processing oneach fraction (e.g., each half) of the symbol period of 2T,computational complexity and costs can be reduced and performance may beimproved. For example, with the present invention, the pilotdemodulation in each assigned correlator (i.e., finger) can be performedbased only on pilot estimates generated by that correlator, whereasconventional techniques may require pilots from multiple correlators.Other advantages are described below.

[0010] An embodiment of the invention provides a demodulator forprocessing a received signal in a wireless communications system. Thedemodulator includes a number of correlators coupled to a combiner. Eachcorrelator typically receives and despreads input samples with arespective despreading sequence to provide despread samples. The inputsamples are generated from the received signal. Each correlator thendecovers the despread samples to provide decovered “partial-symbols” andfurther demodulates the decovered partial-symbols with pilot estimatesto generate correlated symbols. The decovering is performed with achannelization symbol (e.g., a Walsh symbol) having a length (e.g., T)that is a fraction (e.g., half) the length 2T of the channelizationsymbol used to cover the data symbols in the received signal. Thecombiner receives and selectively combines correlated symbols from theassigned correlators to provide demodulated symbols.

[0011] In the transmit diversity mode of a CDMA-2000 or W-CDMA standard(which are identified below), the received signal includes a pair ofsignals transmitted from a pair of antennas. One or more correlators canthen be assigned to process at one or more instances of each transmittedsignal. Each assigned correlator processes the received signal torecover pilot estimates corresponding to the signal instance beingprocessed. The pilot estimates are then used within the assignedcorrelator to demodulate the decovered partial-symbols.

[0012] A specific embodiment of the invention provides a demodulatorthat includes a number of correlators coupled to a combiner. Eachcorrelator typically includes a despreader, a decover element, a complexmultiplier, and a switch coupled in series. The despreader receives anddespreads input samples with a particular despreading sequence toprovide despread samples, and the decover element decovers the despreadsamples to provide pairs of decovered half-symbols. The decovering isperformed with a Walsh symbol W having a length (T) that is half thelength (2T) of a Walsh symbol W_(STS) used to cover the data in thereceived signal. (Space-Time Spreading (STS) is a transmit diversitymode defined by the CDMA-2000 standard.) One pair of decoveredhalf-symbols is provided for each Walsh symbol period of 2T. The complexmultiplier then demodulates the decovered half-symbols with a pilotrecovered by the correlator to provide demodulated half-symbols.

[0013] The switch provides a first combination of decovered half-symbolsfor each Walsh symbol period of 2T in a first (e.g., even) symbol streamand a second combination of decovered half-symbols for each Walsh symbolperiod of 2T in a second (e.g., odd) symbol stream. The combinercombines the first symbol streams from the correlators to provide afirst (even) output symbol stream, and further combines the secondsymbol streams from the correlators to provide a second (odd) outputsymbol stream.

[0014] In one design of this specific embodiment, the multiplier in eachcorrelator performs a dot product and a cross product between thedecovered half-symbols and the pilot to provide “dot” symbols and“cross” symbols, respectively. The combiner can then be designed toselectively combine the dot and cross symbols for each Walsh symbolperiod of 2T to provide the demodulated symbols for the first and secondoutput symbol streams.

[0015] Another specific embodiment of the invention provides ademodulator that also includes a number of correlators coupled to acombiner. Each correlator typically includes a despreader, a decoverelement, first and second summers, and first and second complexmultipliers. The despreader receives and despreads input samples with aparticular despreading sequence to provide despread samples, and thedecover element decovers the despread samples to provide pairs ofdecovered half-symbols. Again, the decovering is performed with a Walshsymbol W having a length (T) that is half the length (2T) of a Walshsymbol W_(STS) used to cover data symbols in the received signal, andone pair of decovered half-symbols is generated for each Walsh symbolperiod of 2T.

[0016] Each correlator typically further includes a switch coupled tothe decover element. The switch provides decovered half-symbolscorresponding to the first half of the Walsh symbol period of 2T to afirst output and decovered half-symbols corresponding to the second halfof the Walsh symbol period of 2T to a second output. Each summer thenoperatively couples to the outputs of the switch and combines each pairof decovered half-symbols in a particular manner to provide a decoveredsymbol. Each multiplier then demodulates the decovered symbols from arespective summer with a respective pilot to provide a respective symbolstream.

[0017] The combiner receives the first and second symbol streams fromthe first and second multipliers, respectively, of each assignedcorrelator, combines the first symbol streams from all assignedcorrelators to provide a first output symbol stream, and furthercombines the second symbol streams from all assigned correlators toprovide a second output symbol stream.

[0018] Another embodiment of the invention provides a method forprocessing a received signal in a wireless communications system. Thereceived signal can include a pair of signals transmitted from a pair ofantennas. In accordance with the method, input samples are generatedfrom the received signal. At least one signal instance of eachtransmitted signal is then processed to provide correlated symbols. Theprocessing for each signal instance typically includes despreading theinput samples with a particular despreading sequence associated with thesignal instance being processed to provide despread samples, decoveringthe despread samples to generate decovered partial-symbols (e.g.,half-symbols), and demodulating the decovered partial-symbols with pilotestimates to generate the correlated symbols for the signal instance.Again, the decovering is performed with a Walsh symbol W having a length(e.g., T) that is a fraction of (e.g., half) the length (2T) of a Walshsymbol W_(STS) used to cover the data in the received signal. Thecorrelated symbols for all signal instances being processed are thenselectively combined to provide demodulated symbols.

[0019] The invention further provides other demodulator architectures,correlators, demodulators, receiver units, and methods to processsignals that have been processed and transmitted in a transmit diversitymode

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The features, nature, and advantages of the present inventionwill become more apparent from the detailed description set forth belowwhen taken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

[0021]FIG. 1 is a simplified block diagram of a communications system inwhich the present invention may be implemented;

[0022]FIG. 2 is a block diagram of a modulator that can be used toprocess a downlink data transmission in a transmit diversity mode inaccordance with CDMA-2000 standard;

[0023]FIG. 3 is a diagram of a complex multiplier;

[0024]FIG. 4 is a block diagram of a conventional demodulatorarchitecture that can be used to demodulate a downlink data transmissionthat has been processed in the transmit diversity mode; and

[0025]FIGS. 5, 6, and 7 are block diagrams of three specific embodimentsof a demodulator architecture of the invention, which are also capableof demodulating the downlink data transmission that has been processedin the transmit diversity mode.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

[0026]FIG. 1 is a simplified block diagram of an embodiment of acommunications system 100 in which the present invention may beimplemented. At a transmitter unit 110, traffic data is sent, typicallyin frames or packets, from a data source 112 to a transmit (TX) dataprocessor 114 that formats, encodes, and processes the data. TX dataprocessor 114 typically further processes signaling and pilot data,which is then combined (e.g., added, or time division multiplexed) withthe processed traffic data to generate composite data. A modulator (MOD)116 then receives, channelizes (i.e., covers), and spreads the compositedata to generate symbols that are then converted to analog signals. Theanalog signals are filtered, (quadrature) modulated, amplified, andupconverted by a transmitter (TMTR) 118 to generate one or moremodulated signals, which are then transmitted via respective antennas120 to one or more receiver units.

[0027] At a receiver unit 130, the transmitted signals are received byan antenna 132 and provided to a receiver (RCVR) 134. Within receiver134, the received signal is amplified, filtered, downconverted,quadrature demodulated, and digitized to provide inphase (I) andquadrature (Q) samples. A demodulator (DEMOD) 136 then receives,despreads, and decovers the samples to generate decovered symbols. Incertain designs, demodulator 136 further demodulates the decoveredsymbols with pilot estimates to generate demodulated symbols. Thedemodulated symbols are then decoded and processed by a receive (RX)data processor 138 to recover the transmitted data. The despreading,decovering, decoding, and processing at receiver unit 130 are performedcomplementary to the spreading, covering, coding, and processing attransmitter unit 110. The recovered data is then provided to a data sink140.

[0028] The signal processing described above supports transmissions ofvoice, video, packet data, messaging, and other types of communicationin one direction. A bi-directional communications system supportstwo-way data transmission. However, the signal processing for the otherdirection is not shown in FIG. 1 for simplicity.

[0029] Communications system 100 can be a code division multiple access(CDMA) system, a time division multiple access (TDMA) communicationssystem (e.g., a GSM system), a frequency division multiple access (FDMA)communications system, or other multiple access communications systemthat supports voice and data communication between users over aterrestrial link.

[0030] The use of CDMA techniques in a multiple access communicationssystem is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREADSPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE ORTERRESTRIAL REPEATERS,” and U.S. Pat. No. 5,103,459, entitled “SYSTEMAND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONESYSTEM”. Another specific CDMA system is disclosed in U.S. Pat. No.6,574,211, entitled “METHOD AND APPARATUS FOR HIGH RATE PACKET DATATRANSMISSION,” issued Jun. 3, 2003. These patents are assigned to theassignee of the present invention and incorporated herein by reference.

[0031] CDMA systems are typically designed to conform to one or morestandards such as the “TIA/EIA/IS-95-A Mobile Station-Base StationCompatibility Standard for Dual-Mode Wideband Spread Spectrum CellularSystem” (hereinafter referred to as the IS-95-A standard), the“TIA/EIA/IS-98 Recommended Minimum Standard for Dual-Mode WidebandSpread Spectrum Cellular Mobile Station” (hereinafter referred to as theIS98 standard), the standard offered by a consortium named “3rdGeneration Partnership Project” (3GPP) and embodied in a set ofdocuments including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS25.213, and 3G TS 25.214 (hereinafter referred to as the W-CDMAstandard), and the “TR-45.5 Physical Layer Standard for cdma2000 SpreadSpectrum Systems” (hereinafter referred to as the CDMA-2000 standard).New CDMA standards are continually proposed and adopted for use. TheseCDMA standards are incorporated herein by reference.

[0032]FIG. 2 is a block diagram of modulator 116, which can be used toprocess a downlink data transmission in a Space-Time Spreading transmitdiversity mode in accordance with the CDMA-2000 standard (hereinafterreferred to as the STS mode). In the STS mode of the CDMA-2000 standard,the data symbols Y to be transmitted are provided to a demultiplexer(DEMUX) 208 and demultiplexed into two complex symbol streams, Y_(even)and Y_(odd), which are then provided to modulators 210 a and 210 b. Theeven complex symbol stream Y_(even) comprises the even inphase symbolstream Y_(I1) and the even quadrature symbol stream Y_(Q1). Similarly,the odd complex symbol stream Y_(odd) comprises the odd inphase symbolstream Y_(I2) and the odd quadrature symbol stream Y_(Q2). The evensymbol streams comprise “even” indexed data symbols and the odd symbolstreams comprise “odd” indexed data symbols. Each modulator 210 performschannelization (i.e., covering) and spreading of the even and odd symbolstreams and provides a complex output symbol stream S for a respectiveantenna.

[0033] In the non-transmit diversity (non-TD) mode of the CDMA-2000standard, complex data symbols are transmitted serially, with each datasymbol having a signaling period of T. In the STS mode, two complex datasymbols are transmitted in parallel over two antennas, with each datasymbol having a signaling period of 2T. As defined by the CDMA-2000standard, within each modulator 210, one of the complex symbol streams(even or odd) is covered with a Walsh symbol W_(STS) having a length of2T, and the other complex symbol stream (odd or even) is covered with acomplementary Walsh symbol {overscore (W)}_(STS) having a length of 2T.

[0034] Within modulator 210 a, the even and odd complex symbol streams,Y_(even) and Y_(odd), are provided to symbol repeaters 212 a and 212 b,respectively. In the STS mode, each symbol repeater 212 repeats eachreceived data symbol once to double the signaling period from T to 2T.The symbol streams from symbol repeaters 212 a and 212 b are thenprovided to cover elements 214 a and 214 b, respectively, which coverthe data symbols with a channelization code associated with the physicalchannel used for the data transmission. In the STS mode, thechannelization code for cover element 214 a is the Walsh symbol W_(STS)having a length of 2T, and the channelization code for cover element 214b is the complementary Walsh symbol {overscore (W)}_(STS) having thesame length of 2T. Each cover element 214 covers (e.g., multiplies) eachreceived data symbol with the Walsh symbol W_(STS) or {overscore(W)}_(STS) in a manner known in the art.

[0035] In the STS mode, the complex symbols from cover element 214 b areprovided to a complex conjugator 216 a that conjugates each receivedsymbol. The conjugated symbols from complex conjugator 216 a are thenprovided to a summer 218 a and subtracted from the symbols from coverelement 214 a to provide complex covered symbols. Each complex coveredsymbol thus includes a pair of data symbols that have been covered withthe Walsh symbols W_(STS) and {overscore (W)}_(STS). The signalprocessing in the STS mode provides diversity in the transmittedsignals, which can result in improved performance.

[0036] In the embodiment shown in FIG. 2, the complex covered symbolsfrom summer 218 a are provided to a phase rotator 222 a. In anembodiment, phase rotator 222 a provides a phase rotation of thereceived complex symbols (e.g., in 90° increments) when enabled by acontrol signal ROTATE. For example, if the received complex symbols areexpressed as I_(C)+jQ_(C), phase rotator 222 a can provide 90° phaserotation of the complex symbols, which can then be expressed as−Q_(C)+jI_(c). The phase rotation allows modulator 210 a to account(i.e., compensate) for phase shifts in the modulated signal due toswitching or adjustments in the subsequent signal conditioning circuitrywithin transmitter 118.

[0037] A complex multiplier 224 a then receives the phase rotatedcomplex symbol stream from phase rotator 222 a and a complex spreadingsequence PN, spreads the complex symbol stream with the complexspreading sequence, and provides a complex output symbol stream S₁. Thecomplex spreading sequence PN is generated in a manner defined by theparticular CDMA system or standard being implemented. For the CDMA-2000system, the complex spreading sequence PN is generated by multiplyingthe short PN sequences, IPN and QPN, assigned to the transmitting basestation with the long PN sequence assigned to the receiving userterminal for which the data transmission is destined.

[0038]FIG. 3 is a diagram of a complex multiplier 300 that can be usedto implement each complex multiplier 224 in FIG. 2. Complex multiplier300 performs a complex multiply of the complex data symbols,D_(I)+jD_(Q), with the complex spreading sequence, PN_(I)+jPN_(Q), toprovide complex spread output symbols, S_(I)+jS_(Q).

[0039] Within complex multiplier 300, the inphase data symbols D_(I) areprovided to multipliers 312 a and 312 b, and the quadrature data symbolsD_(Q) are provided to multipliers 312 c and 312 d. Each of themultipliers 312 a and 312 d also receives the inphase spreading sequencePN_(I), and each of multipliers 312 b and 312 c also receives thequadrature spreading sequence PN_(Q). Each multiplier 312 multiplies thereceived data symbols with the received spreading sequence and providesrespective spread symbols. A summer 314 a receives and subtracts theoutput from multiplier 312 c from the output from multiplier 312 a toprovide the inphase output symbols S_(I). A summer 314 b receives andcombines the outputs from multipliers 312 b and 312 d to provide thequadrature output symbols S_(Q).

[0040] Referring back to FIG. 2, modulator 210 b is configured similarto modulator 210 a, with three differences. First, in modulator 210 b,the complementary Walsh symbol {overscore (W)}_(STS) is used to coverthe even complex symbol stream Y_(even), and the Walsh symbol W_(STS) isused to cover the odd complex symbol stream Y_(odd). Second, complexconjugator 216 b couples to the output of cover element 214 c (i.e., theprocessing path for the even complex symbol stream Y_(even)). And third,the signs for the inputs of summer 218 b are different than the signsfor the inputs of summer 216 a in modulator 210 a.

[0041] The processing performed by modulator 116 can be described asfollows. Initially, the even and odd complex symbol streams can beexpressed as:

Y _(even) =Y _(I1) +j Y _(Q1), and  Eq (1)

Y _(odd) =Y _(I2) +j Y _(Q2).  Eq (2)

[0042] As defined by the CDMA-2000 standard, the Walsh symbols W_(STS)and {overscore (W)}_(STS) are used to cover the even and odd complexsymbol streams. Each of these Walsh symbols has a length of 2T and canbe generated from a Walsh symbol W of length T as follows:

W _(STS) =WW, and  Eq (3) {overscore (W)} _(STS) =W{overscore (W)},where {overscore (W)}=−W.

[0043] If only the data symbols and the covering are considered (i.e.,ignoring the PN spreading, phase rotation, transmit gain, pulse shaping,and other signal processing), the complex output symbol stream forantenna 1 can be expressed as:

S ₁ =Y _(even) WW−Y* _(odd) W{overscore (W)},  Eq (4)

[0044] where the asterisk (*) denotes a complex conjugate operation.Similarly, the complex output symbol stream for antenna 2 can beexpressed as:

S ₂ =Y* _(even) W{overscore (W)}+Y _(odd) WW.  Eq (5)

[0045] The complex output symbol streams, S₁ and S₂, are subsequentlyprovided to two respective processing paths in transmitter 118. Eachprocessing path filters the inphase and quadrature symbol streams, S₁and S_(Q), of the complex symbol stream S, modulates the filtered S₁stream with an inphase carrier signal cos (ω_(c)t), modulates thefiltered S_(Q) stream with a quadrature carrier signal sin (ω_(c)t),sums the two modulated components, and further conditions the resultantsignal to generate a modulated signal. In the STS mode, two modulatedsignals are generated based on two complex symbol streams S₁ and S₂, andare transmitted from two antennas.

[0046] Typically, distinct (i.e., orthogonal) pilots are sent onrespective transmit antennas. For example, for the CDMA-2000 system, anunmodulated pilot (using Walsh code 0, 64) is sent on the common antennaand a modulated diversity pilot (using Walsh code 16, 128) is sent onthe diversity antenna. The pilots are selected to be orthogonal so thatthe amplitude and phase of one or both signals transmitted from therespective antennas can be recovered.

[0047] The downlink signal processing for the CDMA-2000 standard isdescribed in further detail in the CDMA-2000 standard, and by M. Buehreret al. in a paper entitled “Proposed Text for Space Time Spreading (STS)v0.3,” dated 1999, and incorporated herein by reference. This paper wasadopted into the CDMA-2000 standard by the 3GPP2 standard body.

[0048]FIG. 4 is a block diagram of a conventional demodulatorarchitecture 400 capable of demodulating a downlink data transmissionthat has been processed in the STS mode of the CDMA-2000 standard. Inthe STS mode, the received signal includes two modulated signals thathave been transmitted from two transmit antennas. The signal from eachtransmit antenna typically experiences different path conditions, due tothe spatial separation of the transmit antennas, and arrives at thereceiver unit distorted by the particular path conditions. At thereceiver unit, two or more correlators (i.e., fingers) are used toreceive and demodulate the two transmitted signals. The demodulatedsymbols from the correlators are then combined to recover thetransmitted symbols.

[0049] Initially, the received signal is conditioned (e.g., amplified,filtered, downconverted, quadrature demodulated, and so on) anddigitized to provide a complex sample stream comprised of inphasesamples I_(IN) and quadrature samples Q_(IN). The complex sample streamis provided to each correlator assigned to process the received signal.Each correlator receives, tracks, and processes a respective instance(i.e., a particular multipath) of the signal from one of the transmitantennas.

[0050] As shown in FIG. 4, a correlator 410 a is assigned to receive andprocess the signal from the first transmit antenna, and a correlator 410b is assigned to receive and process the signal from the second transmitantenna. Within correlator 410 a, the complex received samples (i.e.,I_(IN)+jQ_(IN)) are provided to a complex multiplier 412 a that alsoreceives a complex despreading sequence PN₁ (i.e., PN₁=PN_(I1)+jPN_(Q1))having a particular time offset assigned to correlator 410 a andmatching the time delay of the signal instance being processed. Complexmultiplier 412 a despreads the complex samples with the PN₁ sequence andprovides the complex despread samples (i.e., I_(D1)+JQ_(D1)) to adecover element 414 a. Decover element 414 a decovers the complexreceived samples with the Walsh symbol W_(STS) and provides complexdecovered symbols to each of the complex multipliers 420 a and 420 b.The decovering is achieved by multiplying the inphase (and quadrature)samples with the Walsh symbol W_(STS) and accumulating the results overthe length (2T) of the Walsh symbol W_(STS) to provide inphase (andquadrature) decovered symbols.

[0051] Complex multiplier 420 a then demodulates the complex decoveredsymbols with a conjugated complex pilot ĥ*₁ (estimated from a pilottransmitted from a first transmit antenna) recovered by correlator 410a. Similarly, complex multiplier 420 b demodulates the complex decoveredsymbols with a conjugated complex pilot ĥ*₂ (estimated from a pilottransmitted from a second transmit antenna) recovered by correlator 410b. The output from complex multiplier 420 a comprises the even complexsymbol stream C_(even) ¹ that is provided to an accumulator 442 a withina combiner 440. Similarly, the output from complex multiplier 420 bcomprises the odd complex symbol stream C_(odd) ¹ that is provided to anaccumulator 442 b within combiner 440.

[0052] Within correlator 410 b, the complex received samples (i.e.,I_(IN)+jQ_(IN)) are despread by a complex multiplier 412 b with acomplex despreading sequence PN₂ (i.e., PN₂=PN_(I2)+jPN_(Q2)) having aparticular time offset assigned to correlator 410 b. The complexdespread samples (i.e., I_(D2)+jQ_(D2)) are decovered by decover element414 b with the complementary Walsh symbol {overscore (W)}_(STS) andconjugated by a complex conjugator 416. The conjugated symbols are thendemodulated with the complex pilot ĥ₂ by a complex multiplier 420 c, andfurther demodulated with the negative complex pilot −ĥ₁ by a complexmultiplier 420 d. The output from complex multiplier 420 c comprises theeven complex symbol stream C_(even) ² that is provided to accumulator442 a, and the output from complex multiplier 420 d comprises the oddcomplex symbol stream C_(odd) ² that is provided to accumulator 442 b.

[0053] Accumulator 442 a combines the even complex symbol streams, andC_(even) ¹ and C_(even) ², from correlators 410 a and 410 b and providesthe even output symbol stream C_(even) (i.e., C_(evenpl =C)_(I1)+jC_(Q1)). Similarly, accumulator 442 b combines the odd complexsymbol streams, C_(odd) ¹ and C_(odd) ², from correlators 410 a and 410b and provides the odd output symbol stream C_(odd)(C_(odd)=C_(I2)+jC_(Q2)). The symbol streams C_(I1), C_(Q1), C_(I2), andC_(Q2) are estimates of the symbol streams Y_(I1), Y_(Q1), Y_(I2), andY_(Q2), respectively, generated within modulator 116 in FIG. 2 andexpressed in equations (1) and (2).

[0054] Demodulator architecture 400 is described in further detail by A.Kogiantis et al. in a paper entitled “Downlink Improvement throughSpace-Time Spreading,” dated Aug. 5, 1999, and incorporated herein byreference. This paper was submitted to the 3GPP2 standard body foradoption into the CDMA-2000 standard.

[0055] Demodulator architecture 400 shown in FIG. 4 has several majordisadvantages. First, sharing of information between correlators isrequired to perform the pilot demodulation. Each correlator 410 performstwo complex multiplications to achieve the pilot demodulation. The firstcomplex multiplication is performed between the decovered symbols andthe complex pilot estimated by that correlator. The second complexmultiplication is performed between the decovered symbols and thecomplex pilot estimated by the other correlator. Demodulatorarchitecture 400 can be modified to share decovered symbols instead ofpilot estimates. However, in both cases, the need to share informationbetween correlators is highly undesirable in many circuit designs.Additional circuitry would likely be required to coordinate the sharingof information, which would lead to increased complexity and costs.

[0056] Second, if more than one multipath of any of the transmittedsignals is processed, it is necessary to pair up correlators with thesame path delay to perform the pilot demodulation. This requirementimposes constraints on the use of the correlators and requirescoordination between the correlators.

[0057] Consequently, as a result of these disadvantages, systemperformance may be compromised by the use of demodulator architecture400.

[0058]FIG. 5 is a block diagram of a specific embodiment of ademodulator architecture 500 of the invention, which is capable ofdemodulating a downlink data transmission that has been processed in theSTS mode of the CDMA-2000 standard. Initially, the received signal isconditioned and digitized to provide a complex sample stream that isprovided to each of correlators 510 a and 510 b. Each correlator 510receives, tracks, and demodulates a signal transmitted from one of thetransmit antennas.

[0059] Within correlator 510 a, the complex received samples (i.e.,I_(IN)+jQ_(IN)) are despread by a complex multiplier 512 a with acomplex despreading sequence PN₁ having a particular time offsetassigned to correlator 510 a. The complex despread samples (i.e.,I_(D1)+jQ_(D1)) are then decovered by decover element 514 a with a Walshsymbol W having a length of T to provide decovered “half-symbols”. Thedecovering is achieved by multiplying the inphase (and quadrature)samples by the Walsh symbol W and accumulating the resultant samplesover the length (T) of the Walsh symbol W.

[0060] Referring back to FIG. 2, in the STS mode, each data symbol iscovered by the Walsh symbol W_(STS) or {overscore (W)}_(STS) having alength of 2T, which corresponds to one STS symbol period. Also,referring to equation (3), the Walsh symbols W_(STS) and {overscore(W)}_(STS) are generated by combining the Walsh symbol W and thecomplementary Walsh symbol {overscore (W)}. The Walsh symbols W and{overscore (W)} each has a length of T, which is half the length of theWalsh symbols W_(STS) and {overscore (W)}_(STS). Each decoveredhalf-symbol from decover element 514 thus corresponds to only half ofthe STS symbol period.

[0061] The complex decovered half-symbols from decover element 514 a areprovided to a switch 520 a. Switch 520 a provides the decoveredhalf-symbols corresponding to the first half of the STS symbol period(switch 520 a in position A) to a delay element 522 a and the decoveredhalf-symbols corresponding to the second half of the STS symbol period(switch 520 a in position B) to summers 524 a and 524 b. Switch 520 acan be implemented with a demultiplexer, registers, latches, or someother element. Delay element 522 a delays the received half-symbols andprovides the delayed half-symbols to summers 524 a and 524 b. The delayis selected such that the decovered half-symbols for each STS symbolperiod are aligned in time at the inputs of each of summers 524 a and524 b.

[0062] For each STS symbol period of 2T (i.e., the length of the Walshsymbols W_(STS) and {overscore (W)}_(STS)) and after the decoveredhalf-symbol corresponding to the second half of the STS symbol periodhas been received, summer 524 a sums the two received half-symbols andprovides the decovered symbol to a complex multiplier 528 a. Similarly,for each STS symbol period, summer 524 b subtracts the half-symbolreceived from switch 520 a from the half-symbol received from delayelement 522 a and provides the decovered symbol to a complex conjugator526 a. Complex conjugator 526 a conjugates the received symbols andprovides the conjugated symbols to a complex multiplier 528 b.

[0063] Complex multiplier 528 a demodulates the complex decoveredsymbols from summer 524 a with a conjugated complex pilot ĥ*₁ recoveredby correlator 510 a. Similarly, complex multiplier 528 b demodulates thecomplex decovered symbols from complex conjugator 526 a with the negatedcomplex pilot −ĥ₁. The output from complex multiplier 528 a comprisesthe even complex symbol stream C_(even) ¹ that is provided to anaccumulator 542 a within a combiner 540, and the output from complexmultiplier 528 b comprises the odd complex symbol stream C_(odd) ¹ thatis provided to an accumulator 542 b within combiner 540.

[0064] Correlator 510 b performs similar processing as correlator 510 a.Within correlator 510 b, the complex received samples (i.e.,I_(IN)+jQ_(IN)) are despread by a complex multiplier 512 b with acomplex despreading sequence PN₂ having a particular time offsetassigned to correlator 510 b. The complex despread samples are thendecovered by decover element 514 b with the Walsh symbol W to providedecovered half-symbols.

[0065] The complex decovered half-symbols from decover element 514 b areprovided to a switch 520 b, which provides decovered half-symbolscorresponding to the first half of the STS symbol period (switch 520 bin position A) to a delay element 522 b and decovered half-symbolscorresponding to the second half of the STS symbol period (switch 520 bin position B) to summers 524 c and 524 d. Delay element 522 b delaysthe received half-symbols and provides the delayed half-symbols tosummers 524 c and 524 d. Again, the delay is selected such that thedecovered half-symbols for each STS symbol period are time-aligned atthe inputs of each of summers 524 c and 524 d. For each STS symbolperiod, summer 524 c subtracts the half-symbol received from switch 520b from the half-symbol received from delay element 522 b and providesthe decovered symbol to a complex conjugator 526 b, which conjugates thereceived symbol and provides the conjugated symbol to a complexmultiplier 528 c. For each STS symbol period, summer 524 d sums the tworeceived half-symbols and provides the decovered symbol to a complexmultiplier 528 d.

[0066] Complex multiplier 528 c demodulates the complex decoveredsymbols from complex conjugator 526 b with a complex pilot ĥ₂ recoveredby correlator 510 b. Similarly, complex multiplier 528 d demodulates thecomplex decovered symbols from summer 524 d with the conjugated complexpilot ĥ*₂. The output from complex multiplier 528 c comprises the evencomplex symbol stream C_(even) ² that is provided to accumulator 542 a,and the output from complex multiplier 528 d comprises the odd complexsymbol stream C_(odd) ² that is provided to accumulator 542 b.

[0067] Accumulator 542 a combines the even complex symbol streams,C_(even) ¹ and C_(even) ², from correlators 510 a and 510 b and providesthe even output symbol stream C_(even) (i.e., C_(even)+C_(I1)+jC_(QI)).Similarly, accumulator 542 b combines the odd complex symbol streams,C_(odd) ¹ and C_(odd) ², from correlators 510 a and 510 b and providesthe odd output symbol stream C_(odd) (i.e., C_(odd)=C_(I2)+jC_(Q2)). Thesymbol streams C_(I1), C_(Q1), C_(I2), and C_(Q2) are estimates of thesymbol streams Y_(I1), Y_(Q1), Y_(I2), and Y_(Q2), respectively,generated within modulator 116 in FIG. 2.

[0068] The processing performed by demodulator architecture 500 can beanalyzed by first characterizing the transmitted symbol streams. Thetransmitted symbol streams, S₁ and S₂, in the STS mode are expressedabove in equations (4) and (5). The Walsh symbols W_(STS) and {overscore(W)}_(STS) of length 2T can each be decomposed into a combination ofWalsh symbols W and {overscore (W)}, each of length T. The transmittedsymbols can be decomposed into a combination of half-symbols transmittedover the first time interval T₁ of the STS symbol period andhalf-symbols transmitted over the second time interval T₂ of the STSsymbol period.

[0069] The transmitted symbols for the first antenna in equation (4) canbe expressed as:

S ₁ =S ₁ ^(T1) , S ₁ ^(T2) , S ₁ ^(T1) =Y _(even) W−Y* _(odd) W, and S ₁^(T2) =Y _(even) W+Y* _(odd) W.  Eq (6)

[0070] Similarly, the transmitted symbols for the second antenna inequation (5) can be expressed as:

S ₂ =S ₂ ^(T1) , S ₂ ^(T2) , S ₂ ^(T1) =Y* _(even) W+Y _(odd) W, and S ₂^(T2) =−Y* _(even) W+Y _(odd) W.  Eq (7)

[0071] The signals from the first and second transmit antennas arereceived with random amplitudes and phases given by the complex valuesh₁ and h₂, respectively. The values h₁ and h₂ characterize the path lossand multipath fading experienced by the transmitted signals. If thenoise is ignored, the composite received signal can be expressed as:

R=S ₁ h ₁ , S ₂ h ₂ =R ^(T1) , R ^(T2) , R ^(T1) =S ₁ ^(T1) h ₁ +S ₂^(T1) h ₂, and R ^(T2) =S ₁ ^(T2) h ₁ +S ₂ ^(T2) h ₂,  Eq (8)

[0072] where R^(T1) and R^(T2) represent the received symbol waveformsfor the first and second time intervals, T₁ and T₂, respectively, of theSTS symbol period. The even complex symbol streams C_(even) ¹ andC_(even) ² from correlators 510 a and 510 b, respectively, can becomputed as: $\begin{matrix}\begin{matrix}{C_{even}^{1} = {( {{\langle{R^{T1},W}\rangle} + {\langle{R^{T2},W}\rangle}} ){\hat{h}}_{1}^{*}}} \\{= {( {{\langle{( {{S_{1}^{T1}h_{1}} + {S_{2}^{T1}h_{2}}} ),W}\rangle} + {\langle{( {{S_{1}^{T2}h_{1}} + {S_{2}^{T2}h_{2}}} ),W}\rangle}} ){\hat{h}}_{1}^{*}}} \\{= {N( {{( {( {Y_{even} - Y_{odd}^{*}} ) + ( {Y_{even} + Y_{odd}^{*}} )} )h_{1}} +} }} \\{ {( {( {Y_{even}^{*} + Y_{odd}} ) + ( {{- Y_{even}^{*}} + Y_{odd}} )} )h_{2}} ){\hat{h}}_{1}^{*}} \\{= {2{N( {{Y_{even}h_{1}{\hat{h}}_{1}^{*}} + {Y_{odd}h_{2}{\hat{h}}_{1}^{*}}} )}}}\end{matrix} & {{Eq}\quad (9)} \\\begin{matrix}{C_{even}^{2} = {( {{\langle{R^{T1},W}\rangle} - {\langle{R^{T2},W}\rangle}} )^{*}{\hat{h}}_{2}}} \\{= {( {{\langle{( {{S_{1}^{T1}h_{1}} + {S_{2}^{T1}h_{2}}} ),W}\rangle} - {\langle{( {{S_{1}^{T2}h_{1}} + {S_{2}^{T2}h_{2}}} ),W}\rangle}} )^{*}{\hat{h}}_{2}}} \\{= {N( {{( {( {Y_{even} - Y_{odd}^{*}} ) - ( {Y_{even} + Y_{odd}^{*}} )} )^{*}h_{1}^{*}} +} }} \\{ {( {( {Y_{even}^{*} + Y_{odd}} ) - ( {{- Y_{even}^{*}} + Y_{odd}} )} )^{*}h_{2}^{*}} ){\hat{h}}_{2}^{*}} \\{= {2{N( {{{- Y_{odd}}h_{1}^{*}{\hat{h}}_{2}} + {Y_{even}h_{2}^{*}{\hat{h}}_{2}}} )}}}\end{matrix} & {{Eq}\quad (10)}\end{matrix}$

[0073] where <R^(T1), W> denotes the decovering of the symbol waveformR^(T1) by the first correlator with the Walsh symbol W, 2N representsthe length of the Walsh symbols W_(STS) and {overscore (W)}_(STS) (inchips), and (AB)*=A* B*. Similarly, the odd complex symbol streamsC_(odd) ¹ and C_(odd) ² from correlators 510 a and 510 b, respectively,can be computed as: $\begin{matrix}\begin{matrix}{C_{odd}^{1} = {{- ( {{\langle{R^{T1},W}\rangle} - {\langle{R^{T2},W}\rangle}} )^{*}}{\hat{h}}_{1}}} \\{= {{- ( {{\langle{( {{S_{1}^{T1}h_{1}} + {S_{2}^{T1}h_{2}}} ),W}\rangle} - {\langle{( {{S_{1}^{T2}h_{1}} + {S_{2}^{T2}h_{2}}} ),W}\rangle}} )^{*}}{\hat{h}}_{1}}} \\{= {- {N( {{( {( {Y_{even} - Y_{odd}^{*}} ) - ( {Y_{even} + Y_{odd}^{*}} )} )^{*}h_{1}^{*}} +} }}} \\{ {( {( {Y_{even}^{*} + Y_{odd}} ) - ( {{- Y_{even}^{*}} + Y_{odd}} )} )^{*}h_{2}^{*}} ){\hat{h}}_{1}} \\{= {2{N( {{Y_{odd}h_{1}^{*}{\hat{h}}_{1}} - {Y_{even}h_{2}^{*}{\hat{h}}_{1}}} )}}}\end{matrix} & {{Eq}\quad (11)} \\\begin{matrix}{C_{odd}^{2} = {( {{\langle{R^{T1},W}\rangle} + {\langle{R^{T2},W}\rangle}} ){\hat{h}}_{2}^{*}}} \\{= {( {{\langle{( {{S_{1}^{T1}h_{1}} + {S_{2}^{T1}h_{2}}} ),W}\rangle} + {\langle{( {{S_{1}^{T2}h_{1}} + {S_{2}^{T2}h_{2}}} ),W}\rangle}} ){\hat{h}}_{2}^{*}}} \\{= {N( {{( {( {Y_{even} - Y_{odd}^{*}} ) + ( {Y_{even} + Y_{odd}^{*}} )} )h_{1}} +} }} \\{ {( {( {Y_{even}^{*} + Y_{odd}} ) + ( {{- Y_{even}^{*}} + Y_{odd}} )} )h_{2}} ){\hat{h}}_{2}^{*}} \\{= {2{N( {{Y_{even}h_{1}{\hat{h}}_{2}^{*}} + {Y_{odd}h_{2}{\hat{h}}_{2}^{*}}} )}}}\end{matrix} & {{Eq}\quad (12)}\end{matrix}$

[0074] The even complex symbol stream C_(even) from combiner 542 a andthe odd complex symbol stream C_(odd) from combiner 542 b can beexpressed as: $\begin{matrix}\begin{matrix}{{C_{even} = {C_{even}^{1} + C_{even}^{2}}},} \\{= {{2{N( {{Y_{even}h_{1}{\hat{h}}_{1}^{*}} + {Y_{odd}h_{2}{\hat{h}}_{1}^{*}}} )}} +}} \\{{{2{N( {{{- Y_{odd}}h_{1}^{*}{\hat{h}}_{2}} + {Y_{even}h_{2}^{*}{\hat{h}}_{2}}} )}},}} \\{{= {2{N( {{Y_{even}( {{h_{1}{\hat{h}}_{1}^{*}} + {h_{2}^{*}{\hat{h}}_{2}}} )} + {Y_{odd}( {{h_{2}{\hat{h}}_{1}^{*}} - {h_{1}^{*}{\hat{h}}_{2}}} )}} )}}},}\end{matrix} & {{Eq}\quad (13)} \\\begin{matrix}{{C_{odd} = {C_{odd}^{1} + C_{odd}^{2}}},} \\{= {{2{N( {{Y_{odd}h_{1}^{*}{\hat{h}}_{1}} - {Y_{even}h_{2}^{*}{\hat{h}}_{1}}} )}} +}} \\{{{2{N( {{Y_{even}h_{1}{\hat{h}}_{2}^{*}} + {Y_{odd}h_{2}{\hat{h}}_{2}^{*}}} )}},}} \\{= {2{{N( {{Y_{odd}( {{h_{1}^{*}{\hat{h}}_{1}} + {h_{2}{\hat{h}}_{2}^{*}}} )} + {Y_{even}( {{h_{1}{\hat{h}}_{2}^{*}} - {h_{2}^{*}{\hat{h}}_{1}}} )}} )}.}}}\end{matrix} & {{Eq}\quad (14)}\end{matrix}$

[0075] In each of equations (13) and (14), the first term is the desiredsignal component and the second term is the undesired component due tocross-talk. If the pilot estimates are accurate (i.e., ĥ₁=h₁ and ĥ₂=h₂),then equations (13) and (14) simplify as follows:

C _(even)=2N Y _(even) (|h ₁|² +|h ₂|²),  Eq (15)

C _(odd)=2N Y _(odd) (|h₁|² +|h ₂|²).  Eq (16)

[0076] Demodulator architecture 500 can recover the transmitted symbolsif one transmit antenna should fail to operate or if the signaltransmitted from one of the antennas experiences a deep fade. As anexample, if the second transmit antenna should fail, the received symbolstream can be expressed as:

R=S₁h₁, R^(T1)=S₁ ^(T1)h₁, and R^(T2)=S₁ ^(T2)h₁.   Eq (17)

[0077] At the receiver unit, one correlator can be used to receive andprocess the transmitted signal. The even and odd complex symbol streams,C_(even) and C_(odd), from the assigned correlator can be expressed as:$\begin{matrix}\begin{matrix}{{C_{even}^{1} = {( {{\langle{R^{T1},W}\rangle} + {\langle{R^{T2},W}\rangle}} ){\hat{h}}_{1}^{*}}},} \\{{= {( {{\langle{( {S_{1}^{T1}h_{1}} ),W}\rangle} + {\langle{( {S_{1}^{T2}h_{1}} ),W}\rangle}} ){\hat{h}}_{1}^{*}}},} \\{{= {{N( {( {( {Y_{even} - Y_{odd}^{*}} ) + ( {Y_{even} + Y_{odd}^{*}} )} )h_{1}} )}{\hat{h}}_{1}^{*}}},} \\{= {2{{N( {Y_{even}h_{1}{\hat{h}}_{1}^{*}} )}.}}}\end{matrix} & {{Eq}\quad (18)} \\\begin{matrix}{C_{odd}^{1} = {{- ( {{\langle{R^{T1},W}\rangle} - {\langle{R^{T2},W}\rangle}} )^{*}}{\hat{h}}_{1}}} \\{= {{- ( {{\langle{( {S_{1}^{T1}h_{1}} ),W}\rangle} - {\langle{( {S_{1}^{T2}h_{1}} ),W}\rangle}} )^{*}}{\hat{h}}_{1}}} \\{= {{- {N( {( {( {Y_{even} - Y_{odd}^{*}} ) - ( {Y_{even} + Y_{odd}^{*}} )} )^{*}h_{1}^{*}} )}}{\hat{h}}_{1}}} \\{= {2{N( {Y_{odd}h_{1}^{*}{\hat{h}}_{1}} )}}}\end{matrix} & {{Eq}\quad (19)}\end{matrix}$

[0078] Again, if the pilot estimate is accurate (i.e., ĥ₁=h₁), thenequations (18) and (19) simplify as follows:

C_(even) ¹=2N Y_(even)(|h₁|²), C_(odd) ¹=2N Y_(odd) (|h₁|²).

[0079] Demodulator architecture 500 shown in FIG. 5 provides a number ofadvantages over demodulator architecture 400 shown in FIG. 4. Theseadvantages can result in a simplified design, reduced costs, improvedperformance, some other advantages, or a combination thereof. Some ofthese advantages are described below.

[0080] First, demodulator architecture 500 does not require the sharingof pilot estimates and data symbols between correlators. Each correlatorreceives, processes, and demodulates the received sample stream with itsown pilot estimate. The autonomous design for the correlators eliminatesthe need to transfer information between correlators and simplifies thedesign of the receiver unit that uses demodulator architecture 500.

[0081] Second, demodulator architecture 500 does not require correlatorsto be paired up. This allows for flexibility in assigning correlators tothe strongest signal instances, which can lead to improved performance.

[0082] Third, demodulator architecture 500 does not requiresynchronization of the pilots of paired correlators that have unequalpath delays. This feature results from the ability of each correlator tooperate independently based on the received samples and its own pilotestimate. In contrast, since the correlators are operated in pairs indemodulator architecture 400, the pilots needs to be properly aligned intime to account for any delays between the signal instances beingprocessed by the pair of correlators.

[0083] Fourth, demodulator architecture 500 allows for reception of thetransmitted symbols if one of the transmit antennas should fail tooperate or is in a deep fade. In contrast, demodulator architecture 400can only recover half of the transmitted symbol should one transmitantenna fail. Demodulator architecture 500 can be used to provide a morerobust and reliable communication.

[0084]FIG. 6 is a block diagram of another specific embodiment of ademodulator architecture 600 of the invention, which is also capable ofdemodulating a downlink data transmission that has been processed in theSTS mode of the CDMA-2000 standard. The complex sample stream isprovided to correlators 610 a and 610 b, with each correlator 610operated to receive, track, and demodulate a signal transmitted from oneof the transmit antennas.

[0085] Within correlator 610 a, the complex received samples aredespread by a complex multiplier 612 a with a despreading sequence PN₁and decovered by a decover element 614 a with the Walsh symbol W toprovide decovered half-symbols. The decovered half-symbols are thendemodulated with a conjugated complex pilot ĥ*₁ recovered by correlator610 a to provide demodulated half-symbols, which are then provided to aswitch 620 a. In the first half of the STS symbol period, switch 620 ais in position A, and the demodulated half-symbol is provided to asignal path 622 a and the inverted demodulated half-symbol is providedto a signal path 622 b. In the second half of the STS symbol period,switch 620 a is in position B, and the demodulated half-symbol isprovided to signal paths 622 a and 622 b. Switch 620 a can beimplemented with a demultiplexer or some other element.

[0086] The demodulated half-symbols on signal path 622 a are provided toan accumulator 642 a within a combiner 640. The demodulated half-symbolson signal path 622 b are provided to a complex conjugator 626 a, whichconjugates the received half-symbols and provides the conjugatedhalf-symbols to an accumulator 642 b within combiner 640.

[0087] Correlator 610 b processes the complex received samples insimilar manner as correlator 610 a. Specifically, correlator 610 bdespreads the complex received samples with a despreading sequence PN₂,decovers the despread samples with the Walsh symbol W to providedecovered half-symbols, and demodulates the decovered half-symbols witha conjugated complex pilot ĥ*₂ recovered by correlator 610 b to providedemodulated half-symbols. The demodulated half-symbols corresponding tothe first half of the STS symbol period are provided to accumulator 642b, and also conjugated and provided to accumulator 642 a. Similarly, thedemodulated half-symbols corresponding to the second half of the STSsymbol period are provided to accumulator 642 b, and also inverted andconjugated and provided to accumulator 642 a.

[0088] For each STS symbol period, accumulator 642 a combines the fourreceived demodulated half-symbols and provides an even output symbol,and accumulator 642 b combines the four received demodulatedhalf-symbols to provide an odd output symbol.

[0089] Demodulator architecture 600 generates equivalent results asdemodulator architecture 500 in FIG. 5. However, by performing the pilotdemodulation after the decovering, only one complex multiplier isrequired. Complex multiplier 616 performs one complex multiply (e.g.,one dot product and one cross product) for each half of the STS symbolperiod (i.e., each period of T). In contrast, each of multipliers 528 indemodulator architecture 500 performs one complex multiply for each STSsymbol period of 2T.

[0090] Also, the summers (i.e., summers 524) used to combine thedecovered half-symbols for each STS symbol period are not needed indemodulator architecture 600 since this function is performed byaccumulators 642 a and 642 b. Each accumulator 642 performs twice thenumber of read-accumulate-write operations for each STS symbol period asaccumulator 542 in demodulator architecture 500.

[0091]FIG. 7 is a block diagram of yet another specific embodiment of ademodulator architecture 700 of the invention, which is also capable ofdemodulating a downlink data transmission that has been processed in theSTS mode of the CDMA-2000 standard. The complex sample stream isprovided to correlators 710 a and 710 b, with each correlator 710operated to receive, track, and demodulate a signal transmitted from oneof the transmit antennas.

[0092] Within each correlator 710, the complex received samples aredespread by a complex multiplier 712 with a despreading sequence PNhaving a particular time offset assigned to that correlator, decoveredby a decover element 714 with the Walsh symbol W to provide decoveredhalf-symbols, and demodulated by a complex multiplier 716 with aconjugated complex pilot ĥ* recovered by that correlator to providedemodulated half-symbols.

[0093] Within correlator 710 a, a switch 720 a provides demodulatedhalf-symbols corresponding to the first half of the STS symbol period toan accumulator 742 a within a combiner 740 and further providesdemodulated half-symbols corresponding to the second half of the STSsymbol period to an accumulator 742 b within combiner 740. Similarly,within correlator 710 b, a switch 720 b provides demodulatedhalf-symbols corresponding to the first half of the STS symbol period toan accumulator 742 c and demodulated half-symbols corresponding to thesecond half of the STS symbol period to an accumulator 742 d. Eachaccumulator 742 selectively combines the received half-symbols toprovide the output symbols.

[0094] In FIG. 7, complex multipliers 716 a and 716 b are eachconfigured to perform two complex multiplies for each STS symbol period.The complex multiply from correlator n for time interval Tx of the STSsymbol period can be expressed as: $\begin{matrix}\begin{matrix}{{C^{n} = {( {X_{I} + {j\quad X_{Q}}} )( {P_{I} - {j\quad P_{Q}}} )}},} \\{{= {( {{X_{I}P_{I}} + {X_{Q}P_{Q}}} ) + {j( {{X_{Q}P_{I}} - {X_{I}P_{Q}}} )}}},} \\{{= {C_{dot}^{n,{Tx}} + {j\quad C_{cross}^{n,{Tx}}}}},}\end{matrix} & {{Eq}\quad (20)}\end{matrix}$

[0095] where X_(I)+jX_(Q) is the complex decovered half-symbol to bedemodulated, P_(I)−jP_(Q) is the conjugated pilot estimate (e.g.,ĥ*=P_(I)−jP_(Q)), and $\begin{matrix}C_{dot}^{n,{Tx}} & \begin{matrix}{and} & C_{cross}^{n,{Tx}}\end{matrix}\end{matrix}$

[0096] are the dot and cross products, respectively, for the complexmultiply.

[0097] As shown in equation (20), each complex multiply can be performedwith a dot product and a cross product. The four complex multipliesperformed by multipliers 716 a and 716 b for each STS symbol period canbe achieved with four dot products and four cross products, which yieldfour “dot” symbols and four “cross” symbols, respectively. The dot andcross symbols are also referred to as intermediate symbols. In anembodiment, the eight intermediate symbols for each STS symbol periodcan be stored to eight memory locations and later combined when thesymbols are retrieved from memory.

[0098] The symbol combination performed by accumulators 742 can becomputed as follows. In correlator 710 a, the dot and cross productsgenerate the intermediate symbols $\begin{matrix}C_{dot}^{1,{T1}} & {and} & {C_{cross}^{1,{T1}},}\end{matrix}$

[0099] , respectively, in the first half of the STS symbol period andthe intermediate symbols $\begin{matrix}C_{dot}^{1,{T2}} & {and} & {C_{cross}^{1,{T2}},}\end{matrix}$

[0100] respectively, in the second half of the STS symbol period.Similarly, in correlator 710 b, the dot and cross products generate theintermediate symbols C_(dot)^(2, T1)  and  C_(cross)^(2, T1),

[0101] respectively, in the first half of the STS symbol period and theintermediate symbolsC_(dot)^(2, T  2)  and  C_(cross)^(2, T  2  ),

[0102] respectively, in the second half of the STS symbol period. Theeven complex output symbols C_(even) can be expressed as:$\begin{matrix}\begin{matrix}{{C_{even} = {C_{even}^{I} + {j\quad C_{even}^{Q}}}},} & \quad \\{{C_{even}^{I} = {C_{dot}^{1,{T1}} + C_{dot}^{1,{T2}} + C_{dot}^{2,{T1}} - C_{dot}^{2,{T2}}}},} & \quad \\{C_{even}^{Q} = {C_{cross}^{1,{T1}} + C_{cross}^{1,{T2}} - C_{cross}^{2,{T1}} + {C_{cross}^{2,{T2}}.}}} & {and}\end{matrix} & {{Eq}\quad (21)}\end{matrix}$

[0103] Similarly, the odd complex output symbols C_(odd) can beexpressed as: $\begin{matrix}\begin{matrix}{{C_{odd} = {C_{odd}^{I} + {j\quad C_{odd}^{Q}}}},} & \quad \\{{C_{odd}^{I} = {{- C_{dot}^{1,{T1}}} + C_{dot}^{1,{T2}} + C_{dot}^{2,{T1}} + C_{dot}^{2,{T2}}}},} & \quad \\{C_{odd}^{Q} = {C_{cross}^{1,{T1}} - C_{cross}^{1,{T2}} + C_{cross}^{2,{T1}} + {C_{cross}^{2,{T2}}.}}} & {and}\end{matrix} & {{Eq}\quad (22)}\end{matrix}$

[0104] To further simplify the computations, equations (21) and (22) maybe expressed as: $\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}{{C_{even}^{I} = {( {C_{dot}^{1,{T1}} + C_{dot}^{1,{T2}} + C_{dot}^{2,{T1}} + C_{dot}^{2,{T2}}} ) - {2C_{dot}^{2,{T2}}}}},} \\{{C_{even}^{Q} = {( {C_{cross}^{1,{T1}} + C_{cross}^{1,{T2}} + C_{cross}^{2,{T1}} + C_{cross}^{2,{T2}}} ) - {2C_{cross}^{2,{T1}}}}},}\end{matrix} \\{{C_{odd}^{I} = {( {C_{dot}^{1,{T1}} + C_{dot}^{1,{T2}} + C_{dot}^{2,{T1}} + C_{dot}^{2,{T2}}} ) - {2C_{dot}^{1,{T1}}}}},}\end{matrix} \\{C_{odd}^{Q} = {( {C_{cross}^{1,{T1}} + C_{cross}^{1,{T2}} + C_{cross}^{2,{T1}} + C_{cross}^{2,{T2}}} ) - {2{C_{cross}^{1,{T2}}.}}}}\end{matrix} & {{Eq}\quad (23)}\end{matrix}$

[0105] In equation (23), the quantity within the parenthesis can becomputed once for the dot products and once for the cross products foreach STS symbol period. Two such combined symbols can be computed foreach STS symbol period. For each output symbol (e.g., C_(even) ¹), acorresponding intermediate symbol (e.g., C_(dot)^(2, T2)

[0106] is scaled by a factor of two (e.g., shifted left by one bit) andsubtracted from a corresponding combined symbol (e.g.,C_(dot)^(1, T1) + C_(dot)^(1, T2) + C_(dot)^(2, T1) + C_(dot)^(2, T2)

[0107] ).

[0108]FIGS. 5, 6, and 7 show three specific embodiments of the presentinvention. Other embodiments can also be designed and are within thescope of the present invention. Generally, the demodulator architecturesof the present invention perform partial processing (e.g., despreading,decovering, pilot demodulation, or a combination thereof) on fractions(e.g., half, quarter, and so on) of the STS symbol period to generateprocessed “partial-symbols”. The processed partial-symbols are thenappropriately further processed and combined to generate the outputsymbols. By performing partial processing on each fraction of the STSsymbol period, numerous benefits described above are achieved.

[0109] The present invention has been described with designs in whichthe partial processing is performed on half-symbols. However, partialprocessing on other fractions of the symbol period may also be performedand are within the scope of the present invention. For example, thepartial processing may be performed on quarter symbol period, eighthsymbol period, or some other fraction.

[0110] In the embodiments shown in FIGS. 5, 6, and 7, two correlatorsare used to process the two signals transmitted from two antennas. Eachof these correlators can be operated to track the timing correspondingto the signal instances being processed.

[0111] The signals from the two transmit antennas may also be processedbased on the same timing (e.g., the timing of one of the signalinstances being processed, or the average timing of the two signalinstances, or others). In this implementation, the same symbols are usedfor both transmitted signals, and the processing can be performed by asingle (modified) correlator. The modified correlator can be designed toperform despreading and decovering with a particular time offset, andtwo pilot demodulation. Common sampling, decimation, despreading, anddecovering are performed for both transmitted signals. The use of thesame timing may result in higher cancellation of cross-talk, which canprovide improved performance.

[0112] The demodulator architectures of the invention can be employed invarious receiver architectures such as, for example, a rake receiver.The design and operation of a rake receiver for a CDMA system isdescribed in further detail in U.S. Pat. No. 5,764,687, entitled “Mobiledemodulator architecture for a spread spectrum multiple accesscommunication system,” and U.S. Pat. No. 5,490,165, entitled“Demodulation element assignment in a system capable of receivingmultiple signals,” both assigned to the assignee of the presentinvention and incorporated herein by reference.

[0113] The rake receiver typically includes many correlators (i.e.,fingers) that are assigned to process strong instances of the receivedsignal. The demodulator architectures of the invention allow for easycombining of symbols or half-symbols from multiple assigned correlators.For example, referring back to FIG. 4, the even complex symbols fromeach assigned correlator are provided to accumulator 442 a and the oddcomplex symbols from each assigned correlator are provided toaccumulator 442 b. For each STS symbol period, each accumulator 442combines all received symbols and provides a complex output symbol.Generally, the rake receiver using the demodulator architectures of theinvention can be designed to include as many correlators as desired.Each accumulator is then designed to accumulate symbols from allassigned correlators.

[0114] The processing to recover the transmitted pilot is known in theart and not described in detail herein. The pilot processing isdependent in the particular CDMA system or standard being implemented.For example, different pilot processing is typically performed dependingon whether the pilot is added to (i.e., superimposed over) the data ortime division multiplexed with the data. An example of the pilotprocessing is described in the aforementioned U.S. Pat. Nos. 5,764,687and 5,490,165.

[0115] For clarity, the demodulator architectures, demodulators, andreceiver units of the invention have been described specifically for theSTS mode defined by the CDMA2000 standard. The invention can also beused in other communications systems that employ the same, similar, ordifferent transmit diversity modes. The demodulator architecture of theinvention can be used to provide the basic functionality (e.g.,decovering, pilot demodulation, and so on). Modification of the basicfunctionality and/or additional processing (e.g., combining, reorderingof the symbols, and so on) may be implemented to achieve the desiredresults.

[0116] For example, the W-CDMA standard provides a space time blockcoding transmit antenna diversity (STTD) mode in which symbols aretransmitted redundantly over two antennas. In the STTD mode, datasymbols are redundantly sent to two modulators, but the symbols providedto the second modulator are reordered, with respect to the symbolsprovided to the first modulator, in accordance with a particularordering scheme. To support the STTD mode, demodulator architectures ofthe invention can be modified to temporarily store the demodulatedsymbols from the assigned correlators, reorder the symbols in theinverse manner, and combine the symbols to recover the transmittedsymbols.

[0117] The demodulator architectures described above can beadvantageously used in a user terminal (e.g., a mobile unit, atelephone, and so on) of a communications system, and may also be usedat a base station. The signal processing for the downlink and uplink maybe different and is typically dependent on the particular CDMA standardor system being implemented. Thus, the demodulator architectures aretypically adopted especially for the particular application for which itis used.

[0118] Some or all of the elements described above for the demodulatorarchitectures of the invention (e.g., the complex multipliers, decoverelements, switches, delay elements, summers, combiner, and so on) can beimplemented within one or more application specific integrated circuits(ASICs), digital signal processors (DSPs), controllers,micro-controllers, microprocessors, programmable logic devices (PLDs),other electronic units designed to perform the functions describedherein, or a combination thereof. Also, some or all of the elementsdescribed above can be implemented using software or firmware executedon a processor.

[0119] As an example, a demodulator can be designed in which thedespreader and decoverer elements for each correlator are implemented inhardware, and the pilot demodulation and symbol accumulation for allcorrelators are performed by a DSP in a time division multiplexedmanner. As another example, one correlator and combiner can beimplemented and used to process samples corresponding to various signalinstances in a time division multiplexed manner. Numerous otherimplementations can be contemplated and are within the scope of thepresent invention.

[0120] The foregoing description of the preferred embodiments isprovided to enable any person skilled in the art to make or use thepresent invention. Various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

What is claimed is:
 1. An apparatus for a communication system,comprising: a decover element for decovering a plurality of receivedsamples to provide decovered half-symbols, wherein the decover elementis configured to perform decovering with a decovering channelizationsymbol having a length (T) that is half the length (2T) of a coveringchannelization symbol used to cover the received samples; and a firstmultiplier for receiving the decovered half-symbols and pilot symbols toprovide demodulated half-symbols.
 2. The apparatus of claim 1, whereinthe received samples are despread received samples, further comprising:a second multiplier for producing the despread received samples.
 3. Theapparatus of claim 1, further comprising: a combiner for combining thedemodulated half-symbols received from the first multiplier.
 4. Theapparatus of claim 3, wherein the combiner comprises: a firstaccumulator for accumulating the demodulated half-symbols correspondingto a first half of a symbol period; and a second accumulator foraccumulating the demodulated half-symbols corresponding to a second halfof the symbol period.
 5. The apparatus of claim 1, further comprising: aswitch for selectively outputting the demodulated half-symbolscorresponding to a first half of the symbol period and the demodulatedhalf-symbols corresponding to a second half of the symbol period.
 6. Anapparatus for a communication system, comprising: a first and secondcorrelator, each correlator including a decover element for decovering aplurality of received samples to provide decovered half-symbols, whereinthe decover element is configured to perform decovering with adecovering channelization symbol having a length (T) that is half thelength (2T) of a covering channelization symbol used to cover thereceived samples; a first multiplier for receiving the decoveredhalf-symbols and pilot symbols to provide demodulated half-symbols; anda switch for selectively sending a demodulated half-symbol along a firstsignal path during a first half of a symbol period and an inverteddemodulated half-symbol along a second signal path during a second halfof the symbol period.
 7. The apparatus of claim 6, wherein the receivedsamples are despread samples, and further comprising: a secondmultiplier for producing the despread received samples.
 8. The apparatusof claim 6, further comprising: a conjugator for conjugating thedemodulated half-symbols along the second signal path.
 9. The apparatusof claim 6, further comprising: a combiner for combining the demodulatedhalf-symbols received from the first and second signal paths of therespective first and second correlators.
 10. The apparatus of claim 9,wherein the combiner comprises: a first accumulator for accumulating thedemodulated half-symbols along the first signal path of the firstcorrelator and the second signal path of the second correlator; and asecond accumulator for accumulating the demodulated half-symbols alongthe second signal path of the first correlator and the first signal pathof the second correlator.
 11. An apparatus for a communication system,comprising: a first and second correlator, each correlator including adecover element for decovering a plurality of received samples toprovide decovered half-symbols, wherein the decover element isconfigured to perform decovering with a decovering channelization symbolhaving a length (T) that is half the length (2T) of a coveringchannelization symbol used to cover the received samples; a switch forselectively outputting the decovered half-symbols corresponding to afirst half of a symbol period along a first signal path and decoveredhalf-symbols corresponding to a second half of the symbol period along asecond signal path; first and second summers, respectively coupled tothe first and second signal paths, for combining each pair of decoveredhalf-symbols to provide a respective decovered symbol; and first andsecond multipliers for receiving the decovered symbols and pilot symbolsto provide demodulated symbols.
 12. The apparatus of claim 11, whereinthe received samples are despread received samples, said first andsecond correlator further including: a third multiplier for producingthe despread received samples.
 13. The apparatus of claim 11, furthercomprising: a conjugator for conjugating the decovered symbols along thesecond signal path of the first correlator and the first signal path ofthe second correlator.
 14. The apparatus of claim 11, furthercomprising: a delay element for delaying the decovered half-symbolsalong the first signal path.
 15. The apparatus of claim 14, furthercomprising: a combiner for combining the demodulated symbols receivedfrom the first and second correlators.
 16. The apparatus of claim 15,wherein the combiner comprises: a first accumulator for accumulating thedemodulated symbols from the first multiplier of the first and secondcorrelators; and a second accumulator for accumulating the demodulatedsymbols from the second multiplier of the first and second correlators.17. A communication system, comprising: a transmitter; and a receiverfor processing a received signal transmitted from the transmitter, saidreceiver including: a decover element for decovering a plurality ofreceived samples to provide decovered half-symbols, wherein the decoverelement is configured to perform decovering with a decoveringchannelization symbol having a length (T) that is half the length (2T)of a covering channelization symbol used to cover the received samples;and a first multiplier for receiving the decovered half-symbols andpilot symbols to provide demodulated half-symbols.
 18. The communicationsystem of claim 17, wherein the received samples are despread receivedsamples, further comprising: a second multiplier for producing thedespread received samples.
 19. The communication system of claim 17,further comprising: a combiner for combining the demodulatedhalf-symbols received from the first multiplier.
 20. The communicationsystem of claim 19, wherein the combiner comprises: a first accumulatorfor accumulating the demodulated half-symbols corresponding to a firsthalf of a symbol period; and a second accumulator for accumulating thedemodulated half-symbols corresponding to a second half of the symbolperiod.
 21. The communication system of claim 17, further comprising: aswitch for selectively outputting the demodulated half-symbolscorresponding to a first half of the symbol period and the demodulatedhalf-symbols corresponding to a second half of the symbol period.
 22. Acommunication system, comprising: a transmitter; and a receiver forprocessing a received signal transmitted from the transmitter, saidreceiver including: a first and second correlator, each correlatorincluding a decover element for decovering a plurality of receivedsamples to provide decovered half-symbols, wherein the decover elementis configured to perform decovering with a decovering channelizationsymbol having a length (T) that is half the length (2T) of a coveringchannelization symbol used to cover the received samples; a firstmultiplier for receiving the decovered half-symbols and pilot symbols toprovide demodulated half-symbols; and a switch for selectively sending ademodulated half-symbol along a first signal path during a first half ofa symbol period and an inverted demodulated half-symbol along a secondsignal path during a second half of the symbol period.
 23. Thecommunication system of claim 22, wherein the received samples aredespread samples, and further comprising: a second multiplier forproducing the despread received samples.
 24. The communication system ofclaim 22, further comprising: a conjugator for conjugating thedemodulated half-symbols along the second signal path.
 25. Thecommunication system of claim 22, further comprising: a combiner forcombining the demodulated half-symbols received from the first andsecond signal paths of the respective first and second correlators. 26.The communication system of claim 25, wherein the combiner comprises: afirst accumulator for accumulating the demodulated half-symbols alongthe first signal path of the first correlator and the second signal pathof the second correlator; and a second accumulator for accumulating thedemodulated half-symbols along the second signal path of the firstcorrelator and the first signal path of the second correlator.
 27. Acommunication system, comprising: a transmitter; and a receiver forprocessing a received signal transmitted from the transmitter, saidreceiver including: a first and second correlator, each correlatorincluding a decover element for decovering a plurality of receivedsamples to provide decovered half-symbols, wherein the decover elementis configured to perform decovering with a decovering channelizationsymbol having a length (T) that is half the length (2T) of a coveringchannelization symbol used to cover the received samples; a switch forselectively outputting the decovered half-symbols corresponding to afirst half of a symbol period along a first signal path and decoveredhalf-symbols corresponding to a second half of the symbol period along asecond signal path; first and second summers, respectively coupled tothe first and second signal paths, for combining each pair of decoveredhalf-symbols to provide a respective decovered symbol; and first andsecond multipliers for receiving the decovered symbols and pilot symbolsto provide demodulated symbols.
 28. The communication system of claim27, wherein the received samples are despread received samples, saidfirst and second correlator further including: a third multiplier forproducing the despread received samples.
 29. The communication system ofclaim 27, further comprising: a conjugator for conjugating the decoveredsymbols along the second signal path of the first correlator and thefirst signal path of the second correlator.
 30. The communication systemof claim 27, further comprising: a delay element for delaying thedecovered half-symbols along the first signal path.
 31. Thecommunication system of claim 27, further comprising: a combiner forcombining the demodulated symbols received from the first and secondcorrelators.
 32. The communication system of claim 31, wherein thecombiner comprises: a first accumulator for accumulating the demodulatedsymbols from the first multiplier of the first and second correlators;and a second accumulator for accumulating the demodulated symbols fromthe second multiplier of the first and second correlators.
 33. A methodfor processing a received signal in a wireless communication system,comprising: decovering a plurality of received samples to providedecovered half-symbols, wherein the decovering is performed with adecovering channelization symbol having a length (T) that is half thelength (2T) of a covering channelization symbol used to cover thereceived samples; and receiving the decovered half-symbols and pilotsymbols to provide demodulated half-symbols by a multiplier.
 34. Themethod of claim 33, further comprising: despreading the receivedsamples.
 35. The method of claim 33, further comprising: combining thedemodulated half-symbols received from the multiplier.
 36. The method ofclaim 35, wherein said combining further comprises: accumulating thedemodulated half-symbols corresponding to a first half of a symbolperiod in a first accumulator; and accumulating the demodulatedhalf-symbols corresponding to a second half of the symbol period in asecond accumulator.
 37. The method of claim 33, further comprising:selectively outputting the demodulated half-symbols corresponding to afirst half of the symbol period and the demodulated half-symbolscorresponding to a second half of the symbol period.